Letters of Intent received in 2016

LoI 2018-1919
GA Symposium: Two decades of accreting millisecond pulsars

Topics

- Current status of what is known about accreting millisecond pulsars

- The difference between pulsating and non pulsating accreting neutron stars

- The difference between transitioning and non-transitioning accreting millisecond pulsars

- Studying dense-matter physics using accreting pulsars

- The reaction of neutron stars to the accretion of matter

- Accretion onto a magnetized neutron star

- Binary evolution of systems harboring an accreting millisecond pulsar

Rationale

Accreting millisecond pulsars are binaries where a very fast (millisecond) spinning low magnetic field (~1E8 Gauss) neutron star is accreting matter from a low-mass donor companion (with typical mass <<1 solar masses). These systems were predicted to exist in 1982 to explain how millisecond radio pulsars are formed due to accretion of matter that would spin up the neutron star. In 1998 the first such system was discovered and since then sixteen more binaries of this kind have been found. Thereafter, accreting millisecond pulsars have provided an incredible body of observational data on extreme phenomena occurring in neutron stars plus a wealth of information on the behavior of accretion onto a magnetized object and binary evolution.

In the last few years accreting millisecond pulsars have also been linked with the new enigmatic behavior of the so-called transitional millisecond pulsars, i.e., accreting millisecond pulsars that turn on as radio millisecond pulsars halting accretion for long periods of time. These discoveries have spurred a great deal of work to better understand the physics behind the back-and-forth state switching that some accreting millisecond pulsars seem to do. For example, what triggers these transitions? Why do some systems go into full outburst, while others appear to be stuck in a much lower luminosity X-ray state?

Besides the accreting millisecond pulsars there are several other types of accreting neutron stars. For example, some accreting X-ray pulsars have a neutron star with very slow spin periods (of order seconds to even thousands of seconds) and strong magnetic field strengths (typically 1E12-1e13 Gauss). In recent years a number of similarities have emerged between the phenomenological behavior of accreting millisecond pulsars and the high field accreting pulsars. Therefore there exists an opportunity to study these questions in a much broader context. For instance, anomalies in the spin and orbital evolution of accreting millisecond pulsars might be linked with similar observations made in high field X-ray pulsars.

Accreting pulsars offer also a unique opportunity to directly study the signals emitted from strongly curved regions of space-time. Indeed the pulsations are thought to be generated on the neutron star surface and the pulse profile shape carries information on the curvature of space-time and on the compactness of neutron stars. There are several ongoing observational efforts (such as ASTROSAT which was recently launched and NICER which is to be launched in 2017), to collect data in the forthcoming years and to then proceed to model the pulse profiles of accreting pulsars. Understanding which systems offer the best opportunities and how to carefully model the observations might be crucial to solve the long-standing problem of the composition of ultra-dense matter, i.e., the material that composes the center of neutron stars.

Other theoretical efforts are instead focused on understanding how the plasma that moves in the accretion flow is affected by the presence of a neutron star magnetosphere. How does the plasma flow along the magnetic field lines? How is the torque transferred from the plasma to the neutron star? Which instabilities develop at the disk/magnetosphere boundary? We are reaching now the point where the complex magneto-hydrodynamic simulations required to address these issues are becoming tractable and finding links between the theoretical results and the various observational manifestations is becoming urgent.

To address all these compelling open questions we propose to organize a symposium during the IAU General Assembly in Vienna in 2018. This would mark the 20 year anniversary of the discovery of the first accreting millisecond pulsar and it will give an excellent opportunity to collect the world experts on these kind of objects in one place to make significant progress in the field and pave the way for the next decade.

Currently SOC members are Rudy Wijnands (the Netherlands, chair), Alessandro Patruno, (the Netherlands, chair), Sharon Morsink (Canada), Alessandro Papitto (Italy), Marina Romanova (USA), Biswajit Paul (India)